View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by PubMed Central Current Neuropharmacology, 2008, 6, 379-385 379 Neuropharmacological Mechanisms Underlying the Neuroprotective Effects of Methylphenidate T.J. Volz* Department of Pharmacology and Toxicology, University of Utah, 30 South 2000 East, Room 201, Salt Lake City, UT 84112, USA Abstract: Methylphenidate is a psychostimulant that inhibits the neuronal dopamine transporter. In addition, methylphe- nidate has the intriguing ability to provide neuroprotection from the neurotoxic effects of methamphetamine and perhaps also Parkinson’s disease; both of which may likely involve the abnormal accumulation of cytoplasmic dopamine inside dopaminergic neurons and the resulting formation of dopamine-associated reactive oxygen species. As delineated in this review, the neuroprotective effects of methylphenidate are due, at least in part, to its ability to attenuate or prevent this ab- normal cytoplasmic dopamine accumulation through several possible neuropharmacological mechanisms. These may in- clude 1) direct interactions between methylphenidate and the neuronal dopamine transporter which may attenuate or pre- vent the entry of methamphetamine into dopaminergic neurons and may also decrease the synthesis of cytoplasmic dopa- mine through a D2 receptor-mediated signal cascade process, and 2) indirect effects upon the functioning of the vesicular monoamine transporter-2 which may increase vesicular dopamine sequestration through both vesicle trafficking and the kinetic upregulation of the vesicular monoamine transporter-2 protein. Understanding these neuropharmacological mechanisms of methylphenidate neuroprotection may provide important insights into the physiologic regulation of dopa- minergic systems as well as the pathophysiology of a variety of disorders involving abnormal dopamine disposition rang- ing from substance abuse to neurodegenerative diseases such as Parkinson’s disease. Key Words: Dopamine, dopamine transporter, methamphetamine, methylphenidate, neuroprotection, neurotoxicity, Parkin- son’s disease, vesicular monoamine transperter-2. INTRODUCTION N O CH3 Methylphenidate (MPD) is a ritalinic acid psychostimu- O lant as shown in Fig. (1). It was first synthesized in 1944 [40] and is sold under several brand names including Ritalin and Concerta. MPD is clinically effective in treating both childhood and adult attention-deficit hyperactivity disorder as well as narcolepsy [7, 18, 35, 36, 53, 55]. The use of MPD MPD CH3 has also been studied and advocated for weaning patients H3C O O from mechanical ventilation [23], for treating giggle inconti- N nence [50], and to ameliorate the psychological distress re- lated to both cancer [11, 60] and human immunodeficiency virus infection [12, 76]. F In the brain, MPD alters the transport of dopamine (DA) across the synaptic plasmalemmal membrane by binding CFT with high affinity to, and thereby competitively inhibiting, Fig. (1). Structures of MPD and the cocaine analog, CFT. The the neuronal DA transporter (DAT) [20, 48, 49, 75]. At dashed box indicates the superposition of the atomic sequence from therapeutic doses of 0.3-0.6 mg/kg, orally administered MPD the anime nitrogen through the ester group in 3-dimensional space. may actually bind to and occupy more than half of the DAT in the human brain [63]. By inhibiting the DAT, MPD in- has very limited affinity for the serotonin transporter [4, 32]. creases extracellular DA concentrations in the brain [55, 61] MPD also binds to both muscarinic and serotonin receptors resulting in a prolonged and/or intensified DA postsynaptic in the brain [32]. signal. This DAT inhibition and change in the signal timing MPD has the intriguing ability to protect against meth- of DA may, at least in part, mediate the behavioral and lo- amphetamine (METH) neurotoxicity. This is evidenced by comotor effects of MPD [41, 49]. Additionally, MPD binds findings that MPD provides complete protection against to and inhibits the neuronal norepinephrine transporter but METH-induced decreases in DA transport when rat striatal synaptosomes are incubated in vitro with both METH and *Address correspondence to this author at the Department of Pharmacology MPD [25]. Additionally, in vivo MPD post-treatment pre- and Toxicology, University of Utah, 30 South 2000 East, Room 201, Salt vents METH-induced persistent decreases in striatal DA Lake City, UT 84112, USA; Tel: (801) 587-9203; Fax: (801) 585-5111; levels, vesicular DA transport, binding of the vesicular mono- E-mail: [email protected] amine transporter-2 (VMAT-2) ligand [3H]dihydrotetrabe- 1570-159X/08 $55.00+.00 ©2008 Bentham Science Publishers Ltd. 380 Current Neuropharmacology, 2008, Vol. 6, No. 4 T.J. Volz nazine (DHTBZ), and vesicular DA content in a rat model of bupropion, and benztropine block or attenuate the neurotoxic METH neurotoxicity [46]. In dopaminergic neurons, METH effects of METH in rat striatum [31]. These authors con- is thought to produce this neurotoxicity by causing excess cluded that “an intact and functional DA uptake site is neces- cytoplasmic DA accumulation which, in turn, increases the sary for the development of METH-induced long-term DA formation of DA-associated reactive oxygen species that depletions.” MPD can also prevent the neurotoxic effects of may overwhelm cellular antioxidant systems [3, 8, 13, 17, 1-methyl-4-phenylpyridinium (MPP+) in both human embry- 66, 69]. onic kidney 293 cells expressing the human DAT and in rat embryonic mesencephalic cultures by blocking the DAT Abnormal cytoplasmic DA accumulation may also con- [30]. tribute to the development of Parkinson’s disease [5, 22], suggesting the possibility that MPD may be neuroprotective In order for this proposed neuropharmacological mecha- in this disease state as well since MPD treatment has been nism of MPD neuroprotection to be valid, METH must be shown to improve motor function in human Parkinson’s pa- transported by the DAT and not diffuse passively across the tients [9]. Additionally, MPD provides protection from the plasmalemmal membrane. Evidence supporting this assump- behavioral and neurochemical effects of 6-hydroxydopamine tion can be obtained from empirical calculation. METH has a in an animal model of Parkinson’s disease [14]. As detailed pKa of 10.1 [1, 42] and, depending upon the pH, can exist in below, these neuroprotective effects of MPD may be due, at either a neutral uncharged form or a cationic form as shown least in part, to its ability to attenuate or prevent abnormal in Fig. (2). The relative abundance of neutral and cationic cytoplasmic DA accumulation in dopaminergic neurons by forms is governed by the Henderson-Hasselbalch equation of modulating the activity of the DAT and the VMAT-2 pH = pKa + log ([neutral]/[cation]). Using this equation and through several neuropharmacological mechanisms. the pKa of METH, approximately 0.2% of METH exists in the neutral uncharged form at a physiological pH of 7.4. The MPD NEUROPROTECTION: DIRECT ACTIONS AT neutral uncharged form of METH is lipophilic with a log P THE DAT value of 2.10 while the cationic form is very unlipophilic The DAT is a plasmalemmal membrane-spanning protein with a log P value of -2.03 [19]. Thus, at physiological pH it that functions to transport extracellular DA back into the is possible that only the fraction of METH which exists in cytoplasm of pre-synaptic dopaminergic neurons. As dis- the neutral uncharged form can passively diffuse across the cussed above, MPD is a DAT inhibitor that competitively plasmalemmal membrane while the METH which exists in binds to a single site on the DAT [48, 49, 75]. MPD shares a the cationic form requires the DAT for transport. This same DAT binding pharmacophore with cocaine analogs and the empirical calculation approach has also recently been used to sequence of atoms from the amine nitrogen through the ester calculate the abundance of amphetamine species that can group of MPD (see Fig. 1) is superimposed in 3-dimensional diffuse across vesicular membranes [73]. space with this same atomic sequence in the cocaine analog, CFT [15]. There are three important types of DAT binding interactions in this shared pharmacophore: a hydrogen bond to the amine nitrogen, one or two hydrogen bonds with the CH3 CH3 ester group, and hydrophobic binding interactions with the NH2 NH non-polar phenyl ring [6]. The hydrogen bonding interac- CH3 CH3 tions with the ester group involve arginine residues on the + DAT [64, 70, 71]. The phenyl ring fits into a hydrophobic H binding pocket on the DAT made up of several amino acid residues. The hydrogen bonding interactions are very spe- Fig. (2). Because the amine nitrogen of METH can accept or donate cific whereas the hydrophobic binding pocket can accept a a proton, METH exists in an equilibrium between a neutral un- variety of atomic geometries among various DAT inhibitors. charged form (shown right) and a positively charged cationic form The phenyl rings of MPD and the cocaine analogs may thus (shown left). fit into the same hydrophobic pocket on the DAT despite their differences in molecular orientation. This is supported Experimental evidence also suggests that METH and by findings that MPD, cocaine, and cocaine analogs do in- other amphetamines may be transported by the DAT. For deed share the same binding site on the DAT [48, 54, 75]. example, amphetamine accumulation into striatal synapto- somes is saturable, temperature-dependent, and inhibited by Through these